Acid dew point corrosion-resistant steel and exhaust gas flow path constituent member

ABSTRACT

A steel has improved sulfuric acid dew point corrosion resistance without relying upon the addition of Sb and can have improved hydrochloric acid dew point corrosion resistance. The acid dew point corrosion-resistant steel includes from 0.005 to 0.200% of C, from 0.20 to 0.80% of Si, from 0.05 to 1.50% of Mn, from 0.002 to 0.020% of P, from 0.005 to 0.015% of S, from 0.10 to 0.50% of Cu, from 0.05 to 0.30% of Ni, from 0.005 to 0.100% of Al, and 0 or more and less than 0.010% of Mo in terms of % by mass, with the balance being Fe and impurities. In the case of attaching great importance to hydrochloric acid dew point corrosion resistance, it is desirable to control the Mo content in the foregoing steel to from 0.005 to 0.030% by mass.

TECHNICAL FIELD

On a surface of a member coming into contact with a gas containing a sulfur oxide or hydrogen chloride, so-called “sulfuric acid condensation” is generated in a lower temperature state than a dew point of the gas. In the case where the instant member is a metal, there is a concern that corrosion proceeds due to condensed water containing sulfuric acid, resulting in a problem. In the present description, such corrosion to be caused due to an acid in condensed water is called “sulfuric acid dew point corrosion”. The present invention relates to a steel having a resistance against the sulfuric acid dew point corrosion given thereto and an exhaust gas flow path constituent member using the same.

BACKGROUND ART

A combustion exhaust gas in a thermal power plant is chiefly constituted of moisture, a sulfur oxide (e.g., sulfur dioxide or sulfur trioxide), hydrogen chloride, a nitrogen oxide, carbon dioxide, nitrogen, oxygen, and the like. In particular, when sulfur trioxide of even 1 ppm is contained in the exhaust gas, a dew point of the exhaust gas often reaches 100° C. or higher, and sulfuric acid condensation is easily generated. For metal members constituting a flow path of such an exhaust gas (for example, members constituting a flue duct wall or chimney, dust collector members, heat exchange members for utilizing heat of an exhaust gas, etc.), it is necessary to apply a material having excellent sulfuric acid dew point corrosion resistance.

PRIOR ART Patent Document

Patent Document 1: JP-B-43-14585

Patent Document 2: JP-A-2003-213367

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

As steels having improved sulfuric acid dew point corrosion resistance, Sb-added steels are known (Patent Documents 1 and 2). However, Sb is an expensive element and becomes a factor to bring about a cost increase of steel materials, and also, in the case where a large amount of Sb is consumed as a steel material raw material, there is uncertainty about raw material procurement. In addition, hot workability of a steel is lowered by the addition of Sb. Furthermore, the toxicity level of Sb on a human body has not necessarily been elucidated yet, and when elution of metal elements due to corrosion is taken into consideration, it is desirable from the standpoint of safety to avoid the use of Sb as far as possible.

Meanwhile, though a stainless steel is generally good in acid resistance, there may be the case where corrosion of the stainless steel proceeds more easily depending upon the concentration of the acid or the temperature as compared with Sb-added steels. That is, not only the stainless steel is expensive, but it may not be said that the stainless steel is a perfect material against the sulfuric acid dew point corrosion.

In view of such present circumstances, in a steel made of ordinary steel as a basis, an object of the present invention is to improve sulfuric acid dew point corrosion resistance without relying upon the addition of Sb, and desirably to further improve corrosion resistance to hydrochloric acid contained in condensed water (hydrochloric acid dew point corrosion resistance).

Means for Solving the Problem

As a result of detailed investigations, the present inventors have found that in a steel having Cu added thereto, when contents of P and S as impurity elements are strictly controlled to specified narrow ranges, the sulfuric acid dew point corrosion resistance can be improved. In addition, it has been noted that in the case of incorporating a trace amount of Mo, corrosion resistance to hydrochloric acid contained in condensed water (hydrochloric acid dew point corrosion resistance) can also be improved without impairing the sulfuric acid dew point corrosion resistance. That is, in a steel composed of general steel component elements which does not contain a special element such as Sb, it has become clear that a “solution” of the component composition range in which the above-described object can be achieved exists. The present invention has been accomplished on a basis of such novel findings.

In order to achieve the above-described object, the present invention provides an acid dew point corrosion-resistant steel comprising from 0.005 to 0.200% of C, from 0.20 to 0.80% of Si, from 0.05 to 1.50% of Mn, from 0.002 to 0.020% of P, from 0.005 to 0.015% of S, from 0.10 to 0.50% of Cu, from 0.05 to 0.30% of Ni, from 0.005 to 0.100% of Al, and 0 or more and less than 0.010% of Mo in terms of % by mass, with the balance being Fe and impurities. In particular, in the case of attaching great importance to the hydrochloric acid dew point corrosion resistance, it is desirable to control the Mo content in the above-described steel to from 0.005 to 0.030% by mass.

In addition, the present invention provides an exhaust gas flow path constituent member that is a member using a steel plate composed of the above-described steel and which in a flow path of a combustion exhaust gas in a coal-burning thermal power plant, constitutes a site at which condensation is generated on a surface thereof upon being exposed to the above-described exhaust gas.

The exhaust gas flow path constituent member as referred to herein means a member constituting a structure of the exhaust gas flow path (for example, a duct, a chimney, etc.) and a member which is disposed within the exhaust gas flow path (for example, a member of a dust collector or heat exchanger). Examples of the member of a heat exchanger include a “cooling fin” installed in a pipe through which a fluid receiving heat flows.

Effect of the Invention

According to the present invention, it has become possible to provide a steel which is improved in the sulfuric acid dew point corrosion resistance, or further in the hydrochloric acid dew point corrosion resistance, without adding Sb. This steel is composed of only generally used steel component elements but does not contain a special element, and therefore, its raw material cost is inexpensive. In addition, a lowering of hot workability to be caused due to the addition of a special element is avoided, too. Furthermore, since Sb which is causing concern about its toxicity on a human body is not used, the present invention is also advantageous from the standpoint of safety. In consequence, the present invention is especially useful for construction of combustion exhaust gas flow path in a coal-burning thermal power plant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating influences of a P content on a corrosion rate in a sulfuric acid aqueous solution.

FIG. 2 is a graph illustrating influences of an S content on a corrosion rate in a sulfuric acid aqueous solution.

FIG. 3 is a graph illustrating influences of an Mo content on a corrosion rate in a sulfuric acid aqueous solution.

FIG. 4 is a graph illustrating influences of an Mo content on a corrosion rate in a hydrochloric acid aqueous solution.

MODES FOR CARRYING OUT THE INVENTION

According to detailed investigations made by the present inventors, the sulfuric acid dew point corrosion resistance can be enhanced by strictly controlling contents of P and S that are impurity elements in a Cu-added steel. In addition, by incorporating a trace amount of Mo, the hydrochloric acid dew point corrosion resistance can be further enhanced, too. Although a mechanism of such enhancement of the sulfuric acid dew point corrosion resistance or hydrochloric acid dew point corrosion resistance has not necessarily been elucidated yet, the following findings are obtained at present.

(1) Cu is effective for formation of a hardly soluble CuS film, and this film increases especially a resistance against sulfuric acid.

(2) Since a decrease of P cleans the ferrite and primary austenite grain boundaries, corrosion of the grain boundaries is suppressed.

(3) Since the quantity of sulfide-based inclusions in the steel is decreased due to a decrease of S, a boundary surface between an easily corrosive inclusion and base iron decreases, and a corrosion rate decreases. However, if the S content is too small, the CuS film is hardly formed, and a corrosion weight loss increases conversely.

(4) When a content of Mo increases, sulfuric acid resistance is lowered. However, the sulfuric acid dew point corrosion resistance is most improved in a range in which a trace amount of Mo is added.

(5) Meanwhile, a corrosion potential is shifted to a noble side by incorporation of Mo, and hydrochloric acid resistance is enhanced. A content range of Mo in which in addition to the sulfuric acid resistance, the hydrochloric acid can also be improved exists.

[Sulfuric Acid Dew Point Corrosion Resistance]

In FIGS. 1, 2 and 3, influences of the P content, the S content, and the Mo content on the corrosion rate in a sulfuric acid aqueous solution are illustrated, respectively. This immersion test is one adopting conditions of a sulfuric acid concentration of 40% by mass, a temperature of 60° C., and an immersion time of 6 hours as very severe conditions assuming a combustion gas of heavy fuel oil (coal). As for the steels used, those in FIG. 1 contain from 0.008 to 0.010% by mass of S; those in FIG. 2 contain from 0.010 to 0.012% by mass of P; and those in FIG. 3 contain from 0.010 to 0.012% by mass of P and from 0.008 to 0.010% by mass of S, and in all of them, the contents of the balance elements other than P, S and Mo all fall within the ranges specified in the present invention.

Under the above-described sulfuric acid immersion test conditions, the corrosion rate of conventional acid dew point corrosion-resistant steels containing Sb, Cu and Mo approximately falls within the range of from 10 to 20 mg/cm²/h. As is clear from FIGS. 1, 2 and 3, excellent sulfuric acid dew point corrosion resistance comparable to the conventional Sb-added steels is obtained in the composition range where the P content is not more than 0.020% by mass, the S content is from 0.005 to 0.015% by mass, and the Mo content is from 0 or more to 0.030% by mass.

[Hydrochloric Acid Dew Point Corrosion Resistance]

In FIG. 4, influences of the Mo content on the corrosion rate in a hydrochloric acid aqueous solution are illustrated. Test conditions are adopted such that a hydrochloric acid concentration is 1% by mass, a temperature is 80° C., and an immersion time is 6 hours. As is clear from FIG. 4, the hydrochloric acid resistance is abruptly improved by the addition of a trace amount of Mo, and when the Mo content is 0.005% by mass or more, the hydrochloric acid resistance becomes good comparably to the conventional Sb-added steels. In consequence, in an application of attaching great importance to simultaneous improvements of the sulfuric acid dew point corrosion resistance and the hydrochloric acid dew point corrosion resistance, taking into consideration the results of FIG. 3 in combination, the Mo content may be controlled to the range of from 0.005 to 0.030.

[Component Elements]

The component elements of the steel of the present invention are described. The term “%” regarding the component elements means % by mass.

C is small in influences on the sulfuric acid dew point corrosion resistance, and in order to ensure the strength as a general structural material, its content is controlled to from 0.005 to 0.200%.

Since Si has an action to enhance the sulfuric acid corrosion resistance, a content of 0.20% or more is ensured. However, the excessive addition of Si lowers descaling properties at the time of hot rolling and brings about an increase of scale defects. Furthermore, Si also becomes a factor to lower welding properties. As a result of various investigations, the Si content is limited to not more than 0.80%.

Since Mn is effective for controlling the strength of steel and also has an action to prevent hot brittleness due to S, a content of 0.05% or more is ensured. It is more effective to make the Mn content to 0.30% or more, and the Mn content may also be controlled to 0.50% or more. However, incorporation of a large quantity of Mn may possibly become a factor to lower the corrosion resistance. The Mn content is tolerated to be up to 1.50%, and it may be controlled to the range of not more than 1.20% or not more than 1.00%.

Since P deteriorates the corrosion resistance, hot workability, or welding properties, its content is limited to not more than 0.020%, and more preferably not more than 0.018%. In order to much more enhance the sulfuric acid corrosion resistance, a decrease of the P content is effective. However, since the excessive decrease of the P content increases the work load in steelmaking and becomes a factor to push costs up, the P content may be controlled to 0.002% or more.

Since S deteriorates the corrosion resistance or hot workability, its content is limited to not more than 0.015%. However, with respect to the sulfuric acid dew point corrosion resistance, it was noted that when the S content is decreased, the corrosion rate conversely turns to an increase (FIG. 2). This may be assumed to be caused due to the matter that in the case of the Cr-free steel that is objective in the present invention, a contribution of the CuS film to an enhancement of the sulfuric acid resistance is considered to be large, and when the S content becomes small, the formation of this CuS film becomes insufficient. As a result of various investigations, it is extremely effective to control the S content to 0.005% or more.

Cu is effective for enhancing the sulfuric acid corrosion resistance, and it is necessary to ensure its content of 0.10% or more. But, since excessive incorporation of Cu becomes a factor to lower the hot workability, the Cu content is limited to not more than 0.50%.

Since Ni has an action to suppress a lowering of the hot workability to be caused due to the addition of Cu, a content of 0.05% or more is ensured. It is more effective to make the Ni content to 0.10% or more. However, since Ni becomes a factor to deteriorate the sulfuric acid corrosion resistance, the Ni content is limited to not more than 0.30%.

Al is an element necessary for deoxidation at the time of steelmaking, and its content is controlled to 0.005% or more. It is more effective to control the Al content to 0.010% or more. However, since Al becomes a factor to lower the hot workability, the Al content is limited to not more than 0.100%.

As described above, Mo is an extremely effective element for enhancing the hydrochloric acid resistance, and therefore, Mo may be added as the need arises in the case of attaching great importance to the hydrochloric acid dew point corrosion resistance. In order to sufficiently exhibit an action to enhance the hydrochloric acid resistance, it is effective to ensure incorporation of Mo of 0.005% or more (FIG. 4). However, since an increase of the Mo content brings about a lowering of the sulfuric acid dew point corrosion resistance, in the case of adding Mo, the addition is conducted within the range of not more than 0.030%. Meanwhile, in order to stably realize particularly excellent sulfuric acid dew point corrosion resistance, it is preferable to control the Mo content to the range of 0 or more and less than 0.010% by mass.

Examples

Steels shown in Table 1 were melted, and hot rolled steel plates (sample materials) having a plate thickness of 2.0 mm were fabricated by a customary method. Using a test specimen cut out from each of the sample materials, a sulfuric acid immersion test and a hydrochloric acid immersion test were conducted under the same conditions (described above) as those in the case of obtaining the plots of FIGS. 1, 2, 3 and 4. In the evaluation of sulfuric acid dew point corrosion resistance, the case where the corrosion rate in the sulfuric acid immersion test is not more than 20 mg/cm²/h was decided as “◯” (good), and other cases were decided as “X” (bad). In addition, in the evaluation of hydrochloric acid dew point corrosion resistance, the case where the corrosion rate in the hydrochloric acid immersion test is not more than 4 mg/cm²/h was decided as “⊚” (excellent), the corrosion rate being more than 4 and not more than 20 mg/cm²/h was decided as “◯” (good), and other cases were decided as “X” (bad).

In addition, a JIS #135 test specimen was fabricated from a cast slab of each of the steels shown in Table 1 and subjected to a high-temperature tensile test at temperatures of three levels of 850° C., 900° C., and 950° C. in conformity with JIS G0567. The test was conducted in the following manner. That is, the whole of a parallel part of the test specimen was heated in the air using an infrared heating furnace; after reaching a prescribed temperature, the test specimen was kept for 10 minutes; and a tensile load was then given at a tensile rate of 5 mm/min, thereby fracturing the test specimen. A temperature of the test specimen was measured with a thermocouple connected to substantially the center of the parallel part and controlled within the range of a prescribed temperature ±10° C.

The case where the fractured surface was ductile at all temperatures of the 3 levels described above was decided as “◯” (hot workability: good), and the case where a brittle fractured surface was perceived at any one of the temperatures was decided as “Δ” (hot workability: slightly bad).

These results are shown in Table 2.

TABLE 1 Chemical composition (% by mass) No. C Si Mn P S Cu Ni Al Mo Others Classification 1 0.230 0.02 0.08 0.010 0.008 0.01 0.02 0.018 — — Comparative example 2 0.044 0.28 0.87 0.011 0.008 0.28 0.12 0.022 — — Inventive example 3 0.041 0.55 0.86 0.009 0.010 0.27 0.14 0.029 — — Inventive example 4 0.041 0.38 0.84 0.004 0.014 0.29 0.15 0.026 — — Inventive example 5 0.044 0.31 0.86 0.007 0.007 0.27 0.14 0.021 — — Inventive example 6 0.046 0.16 0.85 0.013 0.008 0.28 0.15 0.025 — — Comparative example 7 0.160 0.58 0.45 0.010 0.010 0.30 0.16 0.028 — — Inventive example 8 0.008 0.45 0.95 0.012 0.007 0.26 0.14 0.038 — — Inventive example 9 0.041 0.55 0.86 0.009 0.009 0.27 0.14 0.029 — — Inventive example 10 0.043 0.51 0.84 0.002 0.009 0.29 0.15 0.025 — — Inventive example 11 0.054 0.62 0.90 0.028 0.010 0.29 0.15 0.023 — — Comparative example 12 0.049 0.49 0.86 0.005 0.008 0.33 0.11 0.030 — — Inventive example 13 0.043 0.55 0.87 0.013 0.009 0.31 0.15 0.035 — — Inventive example 14 0.041 0.53 0.88 0.020 0.009 0.29 0.15 0.033 — — Inventive example 15 0.039 0.58 0.85 0.011 0.002 0.28 0.13 0.022 — — Comparative example 16 0.041 0.52 0.86 0.011 0.025 0.30 0.14 0.026 — — Comparative example 17 0.035 0.48 0.91 0.010 0.009 0.30 0.16 0.022 — — Inventive example 18 0.043 0.42 0.86 0.011 0.015 0.27 0.14 0.027 — — Inventive example 19 0.052 0.43 0.89 0.014 0.005 0.31 0.14 0.024 — — Inventive example 20 0.043 0.53 0.86 0.012 0.003 0.27 0.14 0.019 — — Comparative example 21 0.031 0.62 0.95 0.011 0.010 0.26 0.14 0.038 0.005 — Inventive example 22 0.041 0.49 0.88 0.010 0.009 0.28 0.15 0.044 0.018 — Inventive example 23 0.026 0.73 0.78 0.010 0.009 0.28 0.13 0.031 0.029 — Inventive example 24 0.028 0.69 0.74 0.011 0.009 0.29 0.15 0.033 0.052 — Comparative example 25 0.033 0.47 0.82 0.011 0.009 0.31 0.15 0.028 0.088 — Comparative example 26 0.040 0.58 0.88 0.011 0.010 0.03 0.13 0.024 — — Comparative example 27 0.048 0.32 0.87 0.011 0.005 0.31 0.02 0.036 — — Comparative example 28 0.032 0.59 0.92 0.011 0.009 0.29 0.36 0.026 — — Comparative example 29 0.045 0.29 0.87 0.010 0.011 0.29 0.14 0.025 0.050 Sb: 0.05 Comparative example Underline: out of the range specified by the present invention.

TABLE 2 Sulfuric acid Hydrochloric acid immersion test immersion test Corrosion rate Corrosion rate Hot No. (mg/cm²/h) Evaluation (mg/cm²/h) Evaluation workability Classification 1 91.0 X 15.3 ◯ ◯ Comparative example 2 15.5 ◯ 10.5 ◯ ◯ Inventive example 3 14.5 ◯ 10.1 ◯ ◯ Inventive example 4 16.1 ◯ 9.9 ◯ ◯ Inventive example 5 14.6 ◯ 8.8 ◯ ◯ Inventive example 6 21.2 X 8.6 ◯ ◯ Comparative example 7 19.5 ◯ 9.3 ◯ ◯ Inventive example 8 16.6 ◯ 8.6 ◯ ◯ Inventive example 9 15.5 ◯ 9.7 ◯ ◯ Inventive example 10 10.2 ◯ 9.2 ◯ ◯ Inventive example 11 23.8 X 11.6 ◯ ◯ Comparative example 12 13.5 ◯ 9.3 ◯ ◯ Inventive example 13 17.1 ◯ 9.8 ◯ ◯ Inventive example 14 19.8 ◯ 8.7 ◯ ◯ Inventive example 15 43.0 X 7.9 ◯ ◯ Comparative example 16 23.5 X 13.4 ◯ ◯ Comparative example 17 17.2 ◯ 9.2 ◯ ◯ Inventive example 18 19.5 ◯ 9.5 ◯ ◯ Inventive example 19 17.3 ◯ 9.6 ◯ ◯ Inventive example 20 32.0 X 9.1 ◯ ◯ Comparative example 21 15.3 ◯ 3.8

◯ Inventive example 22 15.8 ◯ 3.0

◯ Inventive example 23 19.4 ◯ 2.7

◯ Inventive example 24 23.2 X 2.5

◯ Comparative example 25 25.8 X 2.4

◯ Comparative example 26 86.5 X 10.2 ◯ ◯ Comparative example 27 15.8 ◯ 9.6 ◯ Δ Comparative example 28 25.2 X 9.3 ◯ ◯ Comparative example 29 14.2 ◯ 2.9

Δ Comparative example

As is clear from Tables 1 and 2, the steels having a composition specified in the present invention were good in the sulfuric acid dew point corrosion resistance; Nos. 21, 22 and 23 each further containing an appropriate amount of No were also good in the hydrochloric acid dew point corrosion resistance; and all of these were not problematic in the hot workability, too.

On the other hand, though No. 29 containing Sb, Cu, and Mo (corresponding to the conventional acid dew point corrosion-resistant steel) was good in the sulfuric acid dew point corrosion resistance, it was inferior in the hot workability. It is to be noted that No. 27 was inferior in the hot workability because the addition amount of Ni was small. 

1. An acid dew point corrosion-resistant steel comprising from 0.005 to 0.200% of C, from 0.20 to 0.80% of Si, from 0.05 to 1.50% of Mn, from 0.002 to 0.020% of P, from 0.005 to 0.015% of S, from 0.10 to 0.50% of Cu, from 0.05 to 0.30% of Ni, from 0.005 to 0.100% of Al, and 0 or more and less than 0.010% of Mo in terms of % by mass, with the balance being Fe and impurities.
 2. The acid dew point corrosion-resistant steel according to claim 1, wherein the content of Mo is from 0.005 to 0.030%.
 3. An exhaust gas flow path constituent member that is a member using a steel plate comprising the steel according to claim 1 and which in a flow path of a combustion exhaust gas in a coal-burning thermal power plant, constitutes a site at which condensation is generated on a surface thereof upon being exposed to the exhaust gas. 